1. Field of the Invention
This invention relates to an extendable antenna device suitable for small-sized radio devices such as a portable radio telephone set, which is made elastic at a portion mechanically connected to the radio device so as to prevent breakage of the antenna device, as well as to ensure the safety and the convenience of the users.
2. Description of the Related Art
A variety of studies in miniaturization of radio devices have been made together with the spread of portable telephone sets. In order to achieve such miniaturization, the antenna must be accommodated in the radio device at least when the device is not used. The structure of the antenna today is basically of a 5/8.lambda.-whip type, and has an impedance matching unit built in the device, and antenna elements extended from the transmitter or receiver in use.
FIGS. 9 and 10 show a conventional antenna device which extends its antenna elements in use and accommodate them within the device. In FIGS. 9 and 10, reference numeral 1 denotes a housing of a radio device; 2, a whip antenna; and 3, a printed board.
As shown in FIG. 10, whip antenna 2 includes a body 5, a knob 6 attached to a tip of the antenna body, a stop 7 provided at the base of the antenna body, and a housing junction 8 which attaches antenna 2 to housing 1.
Antenna body 5 is made of a flexible material such as piano wire or wire cable so as to absorb an possible external force applied to the antenna body by bending itself elastically. Antenna body 5 extends through a hole 10 in junction 8. Antenna body 5, base 11 and tip 12 are sized so as to fit into hole 10. Provided on junction 8 is a hollow cylindrical support composed of suspended flexible rectangular tongues 13 which hold base 11 and ensure electrical connection between antenna body 5 and junction 8.
In antenna 2, junction 8 is fixedly screwed into housing 1. Antenna 2 and printed board 3 are electrically connected via a high-frequency connector lead 15.
Since whip antenna 2 is attached to housing 1 in the above arrangement, it is accommodatable into housing 1 when it is not used to thereby facilitate its conveyance. Telephone communication is easily achieved when antenna 2 is extended by pulling the knob 6.
In order to completely accommodate antenna 2 into housing 1 in this radio device, it is necessary to reduce the length of antenna 2 compared with the length of housing 1. If the length of antenna 2 is adjusted to the length of the housing 1 of such a radio device which will be miniaturized every year, antenna 2 would become too short, so that the directionality and gain of the antenna are insufficient and hence the radio performance is deteriorated.
It would be conceivable to use a rod antenna which includes a plurality of extendable cylindrical antenna elements instead of a whip antenna, as disclosed in Published Unexamined Japanese Utility Model Application Sho 56-31056. However, since a rod antenna of this type has insufficient flexibility, a load is likely to be applied to the base of the antenna due to vibrations generated during conveyance or installation of a portable radio device which uses the antenna to thereby cause breakage.
In order to eliminate this drawback, there is a method of providing a coil spring at the base of a rod antenna as shown in Published Unexamined Japanese Utility Model Application Sho 57-12567. However, according to this method, the structure itself is complicated, so that manufacture is difficult, and the device is expensive and likely to be broken. Furthermore, according to the particular embodiment, even if the antenna is accommodated, part of the coil spring extends out of the housing to thereby render it inconvenient to carry the device.
As described above, when the conventional whip antenna device is of the type which is accommodated in the housing of the radio device, it cannot be longer than the length of the housing, and hence it cannot obtain a length satisfying an appropriate wavelength for the device.
If the rod antenna is used instead of the whip antenna, it has inconveniences in that it cannot sufficiently withstand vibrations and shocks.